Spray coating with a water-based paint in a spray booth involves generation of large quantities of paint mist particles not deposited on a substrate and these mist particles are generally trapped in a spray booth cleaning water (booth circulating water) as a dilute aqueous solution or dispersion of the paint. While the paint mist so collected in the cleaning water amounts to a substantial quantity, discarding the waste paint-containing water as it is results not only in a loss of the paint but also in pollution problems.
Studies have been undertaken on the recovery and reuse of the paint trapped in cleaning water and Japanese Kokai Publication Sho-49-51324, for instance, describes such a recovery technique. According to this technique, a dilute solution or dispersion of the paint available on entrapment of the spray mist in spray-booth cleaning water is concentrated by means of an ultrafiltration membrane or reverse-osmosis filter membrane to a level comparable to the nonvolatile content of the original water-based paint. Thus, this dilute aqueous paint solution or dispersion is concentrated to separate the water-based paint for reuse.
When the dilute aqueous paint solution or dispersion is concentrated using an ultrafiltration or other membrane filter for recovering the trapped paint from the cleaning water as mentioned above, a large quantity of effluent water (hereinafter referred to as filtrate) is produced. To discard this filtrate, a system for disposal of the waste water will have to be provided. To avoid this, the filtrate is usually returned to the spray booth for reuse as cleaning water.
However, in recycling said filtrate as spray-booth cleaning water, repetition of the recovery of dilute waste paint-water mixture, filtration-concentration thereof and the reuse of the filtrate leads ultimately to an abnormal increase in the viscosity of the paint being concentrated in the paint recovery step so that it becomes increasingly difficult to continue concentration up to a level approximating the nonvolatile matter concentration of the original paint (for example about 50%) and, moreover, it takes a long time for the filtration-concentration to be carried through.
Not fully clear is the mechanism by which the repeated use of the filtrate causes an increase in the viscosity of the paint being recovered but the following mechanism may be tentatively suggested. Thus, a water-based paint contains low molecular weight acid components of diverse origins, such as the unreacted acid monomer originating from the stage of synthesis of the main component resin of the paint, low molecular weight acid-containing polymers secondary produced in the synthesis of the resin, decomposition products of the resin which are formed in the neutralization of a resin varnish with, for example, an amine compound, and products of hydrolysis which are formed in the repeated spray coating, trapping of paint mist in cleaning water, concentration-separation and recycling of the paint under high-temperature conditions, for example in the summer months. When such a dilute solution or dispersion of the water-based paint in cleaning water is filtered, the low molecular weight acid components dissolved in the cleaning water find their way into the filtrate and as the filtration procedure is repeated, the low molecular weight acid components are accumulated in increasing concentrations in the filtrate to be recycled. In the dilute aqueous paint solution or dispersion which is available on trapping the paint mist in such a filtrate rich in low molecular weight acid components, the resin in the paint has been stabilized by such accumulated low molecular acid components but these low molecular weight acid components find their way into the filtrate in the filtration-concentration step for recovery of the paint, with the result that not only the resin in the recovered paint is unstable but the viscosity is increased to detract from the efficiency of filtration, thus making it impossible to carry out concentration up to a level comparable to the nonvolatile matter content of the original paint.
Having been developed under the above circumstances, the present invention has for its object to provide a method of recycling the filtrate available from a waste paint-cleaning water mixture after recovery of the paint without compromise of the concentration efficiency of the paint-water mixture.